Process for irradiating synthetic rubber to improve green strength

ABSTRACT

The green strength of synthetic cis-1,4 polyisoprene is increased to levels equivalent to those of natural rubber by irradiating the polyisoprene with 0.1-6.0 Mrads.

United States Patent Gracia et al.

[ Feb. 22, 1972 [54] PROCESS FOR IRRADIATING SYNTHETIC RUBBER TO IMPROVE GREEN STRENGTH [72] inventors: Albert J. Gracia; Patrick J. Reilly, both of Akron; Sandra J. Walters, Cuyahoga [2]] Appl.No.: 740,459

[52] U.S.Cl. .204] 159.2, 260/94.7 R [51] Int. Cl. ..C06l 1/00, C08d 5/00, C081 [/88 [58] FieldofSearch.... ...........204/l60.l, l59.2',260/94.7

[56] References Cited OTHER PUBLICATIONS Chapiro, Radiation Chemistry of Poiyrnene Systems, Wiley & Sons, 1962) pp. 452- 460.

Pike et al., Jrnl. of Polymer Science, Vol. lX, No. 3 pp. 224- 232 & 244 249.

Primary Examiner-Samuel H. Blech Assistant Examiner-Richard B. Turer Attorney-F. W. Brunner and Richard H. Haas [57] ABSTRACT The green strength of synthetic cis-l,4 poiyisoprene is increased to levels equivalent to those of natural rubber by irradiating the poiyisoprene with 0. i6.0 Mrads.

5 Claims, No Drawings PROCESS FOR IRRADIATING SYNTHETIC RUBBER TO IMPROVE GREEN STRENGTH This invention relates to the physical strength characteristics of unvulcanized rubber compounds. More specifically, it relates to the improvement in the green strength of compounded unvulcanized synthetic and natural polyisoprene rubbers.

Synthetic elastomers have to a major extent supplemented or replaced natural rubber in the fabrication of tires and many other rubber products. More recently stereo-specific polymers, and particularly cis-l ,4 polyisoprene have demonstrated a capability of becoming a complete replacement for natural rubber. One impediment to such complete substitution for natural rubber by synthetic cis-l,4 polyisoprene throughout all types and sizes of tires has been the lack of sufficient green strength and tack for satisfactory building properties in large tires such as those employed by trucks, large earth-moving equipment, etc. The ability to minimize or eliminate this difference between natural rubber and cis-l,4 polyisoprene would materially facilitate its complete substitution for natural rubber.

Green strength is a term commonly employed and well understood by persons in the rubber industry. It is, however, a property difTlcult to define precisely. Basically it is that property of a polymer, most obvious in natural rubber, which contributes to proper building conditions where multiple components are employed and which results in little or no release or relative movement of the assembled components subsequent to assembly and prior to completion of the curing operation. "Tack" is also an important property in the building characteristics of a composite rubber product, but lack of tack can usually be overcome to a large extent by the addition of known tackifying agents. Consequently, any difference in tack between cis-l,4 polyisoprene and natural rubber is ordinarily readily correctable. There has heretofore been a paucity of suitable means for adjusting green strength which would correspond to the use of tackifiers in adjusting tack. Lack of suitable green strength in composite rubber products consisting solely or largely of synthetic polyisoprene has been a significant problem in the rubber industry since the advent of synthetic cis-l ,4 polyisoprene (synthetic natural rubber").

The lnstron green strength test is one measure of the stress/strain properties of unvulcanized compounds. it has been accepted by many as the best indication of the ability ofa compound to resist deformation in the uncured state. Typical stress/strain curves are associated with each elastomer although the magnitude of the curve for a given elastomer will vary depending upon the compounding formulation employed, i.e., depending upon the amount of carbon black, oil, etc., which is added to the elastomer gum stock. However, when employing the same, or even a substantially similar, compound formulation, if the shape and magnitude of the stress/strain curves of two clastomers are comparable, the elastomers will possess equivalent building and handling characteristics.

The normal stress/strain curve of raw polymers or unvulcanized compounds shows a definite inflection (yield point) in the stress at relatively low strain. After this initial stress inflec' tion is passed, continued elongation may cause the stress to (I) continue to increase at a different rate, (2) stay nearly constant, or (3) continually fall until rupture of the specimen occurs. Which of the three phenomena occurs is dependent on the type polymer, the amount and nature of the other compounding ingredients in the recipe and the amount and nature ofmastication.

It has been found that the performance of a green com pound in an unvulcaruzed tire can be predicted by three points of the stress/strain curve, (1) the first peak. or inflection, of the stress, (2) the ultimate or breaking tensile, (3) the percen'f' ultimate elongation. Improvements in one or more of the stress properties indicate improved green strength.

Applicants have now discovered a process for improving the prises irradiating synthetic cis-l,4 polyisoprene and sub sequently masticating the irradiated polymer.

The synthetic cis-l,4 polyisoprene may be any such polyisoprene regardless of the catalyst system employed in its polymerization. The invention, however, is especially useful when employed with the high" cis-l,4 polyisoprene formed when a catalyst system such as an Al-Ti or a Li based system is employed. The Al-Ti system consists of an aluminum alkyl compound (such as an aluminum trialkyl or an aluminum tri alkyl complex formed from the reaction of an aluminum trialkyl and an aromatic ether such as diphenyl ether or anisole) in mixture with a transition metal halide (such as titanium tetrachloride) at about equal mole ratios or slightly less of the aluminum to the titanium. The Li based catalyst comprises a or may be a cement. If the latter, any conventional solvent may be employed and the solids content, while not critical, will normally be in the range of lO-ZO percent.

The irradiation source may be an electron accelerator or an isotope source such as gamma radiation from Co". Where electron accelerators are employed, the polymer must be presented in a thin layer because of the well known relatively low penetration as compared to that produced by an isotope source.

The irradiation dose rate should be in the range between 0.0l and L000 Mradslhour and preferably between 0.01 and lM." t-....

The irradiation dose should be in the range between ().I and 6.0 Mrads and preferably between 0.5 and 3.0 Mrads. When the dose. is increased beyond 3.0 Mrads, the physical proper ties are affected so that the processing of the polymer becomes difficult. With dosages in excess of 6.0 Mrads the polymer frequently has unacceptable processing charactownie mm r mor Masticatioii is accomplished on any conventional equip ment suitable for that purpose and in the normal manner well known to those skilled in the art. However, mastication is necessary to achieve the benefit of the invention. Irradiated .gum stocks show little difference in green strength, as determined by the lnstron test, until after mastication.

EXAMPLES The following examples are set forth to illustrate the invention:

The samples (solid or cement) were irradiated by using a C0 source at a dose rate of 0.l Mrad/hour. Samples were masticated eight minutes at 225 F. and rpm. In a 'Brabender Plastograph. All tensile values shown are after mastication, and were obtained by pulling 6 inch dumbbells at l0 incheslminute on a conventional lnstron testing machine Blends were prepared by loading precalculated amounts of the two substituents into the Brabender chamber and masticating eight minutes at 225 F. and 50 rpm Table I shows the after mastication effect of various irradiation doses on both solid and cement samples.

Table II shows the effect of blending irradiated and nonir- 'radiated samples.

Table Ill shows the effect of mastication on preirradiated green strength of synthetic cis-l,4 polyisoprene which comsamples.

TABLE II Mastinateri blends oi irradiated and non-irradiated synthetic (is-1,4 pnlyi onrr'nc Instron tensiles The average radiation dose was calculated [tom the weight percentage of irradiated rubber in the blend.

TABLE III irradiated cis-1,4 polylsoprene before and after mastication Unmastieated Masticated lnstron tensiles instron tenslles Break Break Polymer Radiation Yield Yield state during dose 1 point Percent point Percent irradiation (Mrad (p.s.i.) Psi. along. (psi) P.s 1 along.

Cement None 42 50 1, 600 27 8 Z. 000 0. 4 i3 66 1, 900 27 49 1,100 0.7 43 74 600 26 57 J 1. 0 39 56 400 26 62 800 2. 0 36 53 250 26 80 400 Solid None 44 72 1, 500 26 8 2. 500 0. 4 45 77 660 26 11 2, 500 0. 7 45 72 B00 26 15 2, 300 1. 0 44 66 400 24 2'2 2. L00 2. D 38 48 500 23 all 1, 600

l Doses based on total cement. In terms 0! solid rubber it would be 5-6 times the given Zalue.

TABLE 1 lnstron Tensile Data for Irradiated Masticatcd Synthetic Cis-l .4 Polyisoprene Gum Stocks (llThc control had a 5.0 DSV and I39 gel unmaslicated: a 3.5 DSV and 7 percent gel after mastication. The irradiated samples were used for the blends shown in Table ll.

[2) Doses based on total cement in terms ot solid rubber it would be 5-6 times the givtn value. The control had a 3.5 DSV and l2 percent gel unmasticated; a 2.3 DSV and 4 percent gel after mastication Applicants process is useful in producing synthetic cis-l ,4 polyisoprene which more closely approximates natural rubber. The products resulting from use of the process may be employed in any of the well known uses of natural rubber.

While certain representative embodiments and details have been shown for the purpose ot'illustruting the invention. it will be apparent to those skilled in the art that various Changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed is: l. The process of (l) irradiating synthetic cis-l.4

polyisoprene containing at least percent cis-l,4 content with a dose of between 0.1 and 6.0 Mrads and (2) musticuting the irradiated polymer in any conventional manner.

2. The process according to claim 1 wherein polyisoprene is formed by using an Al-Ti catalyst system.

3. The process according to claim 1 wherein the close is between 0.5 and 3.0 Mrads. V 7

4. The process according to claim 1 wherein the average dose of a blended polymer consisting of between 1:5 and parts by weight of irradiated and nonirradiatcd synthetic cis 1,4 polyisoprene is between 0.5 and 3.0 Mrads.

S. The process of claim 1 wherein the polyisoprcnc contains at least 90 percent cits-1.4 addition, the dose is between 0.5 and 3.0 Mrads. the dose rate is between 0.01 and l0.0 Mrads/hour, and the irradiation source is an isotope.

t i i i 0 the 

2. The process according to claim 1 wherein the polyisoprene is formed by using an A1-Ti catalyst system.
 3. The process according to claim 1 wherein the dose is between 0.5 and 3.0 Mrads.
 4. The process according to claim 1 wherein the average dose of a blended polymer consisting of between 1:5 and 1:0 parts by weight of irradiated and nonirradiated synthetic cis-1,4 polyisoprene is between 0.5 and 3.0 Mrads.
 5. The process of claim 1 wherein the polyisoprene contains at least 90 percent cis-1,4 addition, the dose is between 0.5 and 3.0 Mrads, the dose rate is between 0.01 and 10.0 Mrads/hour, and the irradiation source is an isotope. 